1. Field of Invention
The present invention relates to a planar antenna. More particularly, the present invention relates to a planar antenna for a radio frequency identification (RFID) tag.
2. Description of Related Art
In recent years, RFID systems having the advantages of contactless identification, data security, and being capable of simultaneously reading multiple tags, have gradually replaced the current bar code tag systems. The RFID systems can be applied in very wide fields, such as access control cards, easy cards, and animal identification chips, and can be further applied in fields of logistics management, book management, and medical and drug administration, and so on.
An RFID system mainly comprises a reader system and a tag system. The reader system transmits the tag information to the tag system through an electromagnetic signal. The tag system receives or transmits the electromagnetic signal with a planar antenna, and discriminates the tag information carried by the electromagnetic signal with a tag chip, so as to decide whether or not to transmit the tag information back to the reader system. During the transmission of the tag information between the reader system and the tag system, whether the tag system has enough power to operate or whether the tag information can be transmitted back to the reader system depends on the conjugate match between the planar antenna and the tag chip. When the planar antenna and the tag chip have a good conjugate match, a maximum power transfer occurs between the planar antenna and the tag chip.
Generally, the tag system adopts a dipole antenna as the planar antenna. FIG. 1 is a schematic structural view of a conventional planar antenna. The conventional planar antenna 100 comprises a dielectric slab 110 and a dipole antenna 120, wherein the dipole antenna 120 is disposed on a surface 111 of the dielectric slab 110. However, as the size of the dipole antenna 120 of the conventional planar antenna 100 is too large, the miniaturization cannot be achieved.
In order to solve the above problem, an innovative planar antenna 200 adopts a dipole antenna and a fractal structure in the design of the planar antenna. FIG. 2 is a schematic structural view of the innovative antenna. In order to achieve the object of miniaturization, the innovative planar antenna 200 fragments the dipole antenna 120, such that this fractal dipole antenna has a fractal structure formed by fragmentation. The fractal structure refers to that the innovative fractal dipole antenna 210 is constituted of a plurality of sub-radiating elements (e.g., the sub-radiating elements 211-216 as shown in FIG. 2), and each of the sub-radiating elements has the same geometrical shape (e.g., the equilateral triangle). The width and height of the innovative fractal dipole antenna 210 are indicated by arrows as shown in FIG. 2. However, though the innovative planar antenna 200 is miniaturized, as the concept of the complex conjugate match is employed, the innovative planar antenna 200 cannot have a good conjugate match with the tag chip.
In view of the above, the innovative planar antennae cannot achieve the miniaturization and the conjugate match at the same time. In other words, when an RFID system is used with the innovative planar antenna which is not of conjugate match, the coverage of the identification distance thereof is limited, thus, they cannot be applied in systems practically.